Resistance wire snap acting circuit breaker switch



United States Patent 3,340,374 RESISTANCE WIRE SNAP ACTING CIRCUIT BREAKER SWITCH Tokuo Shiraishi and Yozo Shioda, Tokyo, Shiro Yamauchi, Kodaira-shi, and Shigeru Terada, Tokyo, Japan, assignors to Nikko Denki Seisakusho, Ltd., Tokyo, Japan, a corporation of Japan Filed Sept. 8, 1965, Ser. No. 485,845 Claims priority, application Japan, Dec. 30, 1964, 39/74,500; (utility model), 39/102,431 1 Claim. (Cl. 200-113) This invention relates to a novel type of overload protection device for use in an electric circuit.

Overload protection devices hitherto known in the art have mostly made use of the flexion or bending action of bimetal elements which tend to bend when heated. As is commonly known, these conventional overload protection devices have been defective in that the bimetal is susceptible to the effect of external air temperature and a complex structure is generally required for the means to provide protection against short-circuit current.

It is therefore the primary object of the present invention to provide an overload protection device which utilizes expanding movement of a hot wire for the interruption of overcurrent and which is thus free from the prior defects as described above.

According to the present invention, there is provided an overload protection device comprising a base plate of electrically insulating material, terminals provided on said base plate, a first stationary contact connected with'one of said terminals, a second contact movable towards and away from said first contact, a movable strip of electrically conductive resilient material carrying said second contact on one end thereof and normally biased to bend in one direction, and a hot wire of electrically resistive material connected at one end thereof to that end of said movable strip carrying said second contact thereon and connected at the other end thereof with the other terminal to form an electrical circuit, said hot wire being so tensioned as to bend said movable strip in a direction opposite to its normally biased bent direction to provide pressure contact between said first and second contacts, whereby thermal extension of said hot wire due to heat generated by overcurrent can invert the direction of bend of said movable strip to urge said sec-0nd contact away from said first contact for thereby interrupting the current.

The above and other objects, advantages and features of the present invention will become apparent from the following description with reference to the accompanying drawings, in which:

FIG. 1 is a fragmentarily vertical sectional front elevation of one embodiment of the device according to the invention, the view showing the device in its closed contact position;

FIG. 2 is a view similar to FIG. 1, but showing the device in its opened contact position;

FIG. 3 is a plan view of a movable strip used in the device of FIG. 1;

FIG. 4 is a plan view of another movable strip also used in the device of FIG. 1;

FIG. 5 is a fragmentarily vertical sectional front elevation of another embodiment according to the invention, the view showing the device in its closed contact position; FIG. 6 is a view similar to FIG. 5, but showing the device in its opened contact position;

FIG. 7 is a plan view of the device of FIG. 5, with a protecting cover removed to show the internal structure.

Referring to FIGS. 1 to 4, the first embodiment of the present invention will be described. The overload protection device includes a base plate 1 of electrically insulating material such as Bakelite. A hot wire or resistance wire shown in FIG. 1, the

2 has one end thereof welded or otherwise fixed to a point substantially intermediate the web portion of a U-shaped movable strip 4 of electrically conductive resilient material as shown in FIG. 3.

A movable contact S is securely fixed on the web portion of the movable strip 4 at a point adjacent the fixed end of the hot wire 27 The resilient movable strip 4 is so biased that it normally takes a downwardly bent shape as shown by a solid line in FIG. 2, and a hole 4' is provided at the end of each leg portion so that the movable strip 4 can be secured on the base plate 1 as by rivets 7 with a block 6 of electrically insulating material interposed therebetween. A stationary contact S has its supporting extension extended downwardly and riveted to the base plate 1, and its lower end projects through the base plate 1 to form a terminal L A U-shaped movable strip 5 of resilient metal material, which is smaller in shape than the movable strip 4, has also its ends bored as at 5' so as to be securely fixed on the insulating block 6 by the rivets 7 in overlapped relation on the movable strip 4.

The free end of the movable strip 5 is suitably spaced from the movable strip 4 and has two engaging heads 8 formed thereat as shown in FIGS. 1 and 3. The other or free end of the hot wire 2 is securely fixed on the insulating block 6 and is extended downwardly for connection with a terminal L A protecting cover 9 covers the above-described elements and a resetting push button 3 loaded with a compression spring 10 extends through a substantially central portion of the top wall of the protecting cover 9.

In the device of the invention, the hot wire 2 is passed in tension so that the U-shaped, resilient, movable strip 4 fixed on the insulating block 6 is bent in a direction opposite to its normally biased direction whereby suflicient pressure contact between the stationary contact S and the movable contact S, can always be maintained under the normal operative state, as shown in FIG. 1, in which no overload takes place.

Suppose overcurrent develops during conduction of current through the electric circuit consisting of terminal L hot wire 2contact S contact S terminal L in FIG. 1. Then, Joule heat developed in the hot wire 2 causes gradual generation of heat and extension of the hot Wire 2. When finally the hot wire 2 is extended over a certain predetermined value, the resilient movable strip 4 is urged beyond its dead point and bounces abruptly back to its normally downwardly biased state to break the contact between the contacts 8, and S as shown in FIG. 2. This working point responsive to overload current can suitably be determined by adjusting the tension on the hot wire 2.

In resetting the device at the normal operative state as push button 3 may merely be depressed to urge the U-shaped, resilient, movable strip 5 onto the resilient movable strip 4 under the state as shown in FIG. 2. In the course of depression, the heads 8 of the movable strip 5 engage the upwardly bent leg portions of the resilient movable strip 4 to force the latter downwardly and the strip 4 is oppositely urged beyond its dead point to bounce abruptly back to the state as shown by dotted line in FIG. 2. When the force on the push button 3 is released, the button 3 returns to its original position by the force of the spring 10, while the resilient strips 4 and 5 return to their original positions by their own resiliency so that the contact S is again brought into firm contact with the contact S to complete the circuit. It will be known that, the moment the push button 3 is depressed for the resetting of the device, the resilient strip 4 is turned over from the state as shown by solid line in FIG. 2 to the state as shown by dotted line, in which state the movable contact S is not yet in contact with the stationary contact S but as the force on the push button 3 is slowly relieved to move the button 3 gradually upwardly, the contact S gradually approaches the contact S until finally they are brought into firm contact with each other to complete the circuit. An important feature of the invention is that the circuit breaking action responsive to overload current as described above can likewise be effected even in such a state. Thus, the present invention provides a trip free type of overload protection device which can freely be reset even in a circuit under continuous power supply.

Referring next to FIGS. 5 to 7 another embodiment of the present invention will be described. The overload protection device in this embodiment includes a base plate 101 of electrically insulating material such as Bakelite. A movable strip 103 of electrically conductive resilient material is so biased as to normally bend upwardly as shown in FIG. 6 and has a contact S firmly secured at a position substantially intermediate its length. A strip 104 of substantially U-shaped section of resilient metal material has near one end thereof firmly fixed a contact S in opposed relation to the contact S and has the other end thereof fixed together with a terminal strip L to the base plate 101 as by a rivet. The movable strip 103 is securely received at opposite ends thereof in respective holes provided near substantially V-shaped free ends of receiving members 112 and 113 of resilient metal material which are fixed at lower ends to the base plate 101 as by rivets. The receiving members 112 and 113 are normally urged outwardly by their own resiliency.

A hot wire 102 in the form of a wire or band is welded or otherwise fixed at one end thereof to a portion 110 substantially intermediate the length .of the receiving member 113, while the other end of the hot wire 102 is welded or otherwise fixed to an extension 111 of a terminal strip L which is secured in the base plate 101. A resetting push button 106 loaded with a compression spring 107 is mounted on a protecting cover 105 and its lower extension 109 extends downwardly through the top wall of the cover 105 and a hole 114 bored through the resilient strip 103. A stopper 115 prevents the push button 106 from escaping out of the protecting cover 105.

In the present embodiment of the invention, the hot wire 102 is passed in tension so that the resilient strip 103 normally biased to bend upwardly as shown in FIG. 6 is oppositely bent until the movable contact S is brought into pressure contact with the stationary contact S as shown in FIG. 5 and under this state current is allowed to flow through the circuit consisting of terminal L extension 111hot wire 102-portion 110-receiving member 113resilient strip 103contact S contact S -resilient strip 104-terminal L Suppose overcurrent develops during conduction of current through this circuit. Then, Joule heat developed in the hot wire 102 causes gradual generation of heat and extension of the hot wire 102. When finally the hot wire 102 is extended over a certain predetermined value, the resilient metal strip 103 is urged beyond its dead point and bounces abruptly back to its normally upwardly biased state to break the contact between the contacts S and S as shown in FIG. 6. This working point responsive to overload current can suitably be determined by adjusting the tension on the hot wire 102.

In resetting the device at the normal operative state as shown in FIG. 5, the push button 106 may merely be depressed to urge the U-shaped resilient strip 104 having the stationary contact S downwardly by the lower extension 109 of the push button 106. Then, a shoulder 108 on the push button 106 is engaged by the edge of the hole 114 and urges the resilient strip 103 downwardly beyond its dead point so that the resilient strip 103 is abruptly urged to its downwardly bent position. When subsequently the force on the push buton 106 is slowly relieved, the push buton 106 moves upwardly by the force of the compression spring 107 with the result that the U-shaped resilient strip 104 having been urged downwardly returns to its original position and the contacts S and S retake the position, as shown in FIG. 5, at which they are in pressure contact with each other.

The device according to the invention is advantageous over conventional devices in that it can easily interrupt a great current such as short-circuit current since less time is required for the interruption due to the fact that the hot wire is heated at a higher rate than the bimetal.

Further, by arranging in a manner that the direction of current flow through the resilient strip 103 is opposite to the direction of current flow through the resilient strip 104 as shown by arrows in FIG. 5, the force of electromagnetic repulsion generated therebetween effects a snap action and thus the device can be used with a circuit with a greater short-circuit current. The present invention thus provides a trip free type of overload protection device in which the stationary contact S is not brought into pressure contact with the movable contact S during the resetting operation unless the force on the push button 106 is completely removed, although the resilient strip 103 has been bent downwardly by being urged by the push button 106 and, once the contacts are brought into contact with each other, the device is ready to make the subsequent operation. In other words, the structure of the device is such that the device is ready to make current interrupting operation even when it is reset in a shorted circuit.

It is a feature of the present invention that the inventive device is affected less by external air temperature than in conventional bimetal type devices since a hot wire being a resistance wire is directly heated to extend and a considerably high temperature thereby developed provides a great difference between it and external air temperature. It is another feature that the hot wire of small cross-sectional area radiates heat quickly and is cooled down in a very short period of time and thus immediate resetting is made possible whereas the bimetal once heated requires a considerable time before it is cooled down.

From the foregoing description, it will be understood that, in the present invention, thermal extension of hot wire due to Joule heat is utilized and a metal strip held in its bent state is turned into an oppositely bent state beyond its dead point by the slight extension of the hot wire. This arrangement insures a very high sensitivity. Further, a hot wire of short length will quite suffice and hence a small amount of heat will also suflice to place the device in operation. Thus, the device can have a small shape and a simple structure as a whole and can be manufactured at low cost.

What is claimed is: A circuit breaker comprising a base of insulating material, first and second electrical terminal members secured to said base, a first contact resiliently mounted on said base and electrically connected to said first terminal, a pair of resilient support members attached to said ase, an upwardly convexed biased spring resilient strip supported between said resilient support members and having a second contact mounted thereon in opposed relation to said first contact, a resistance wire connected in tension between said second electrical terminal and one of said pair of resilient support members positioned to pull said.

pair of resilient support members toward each other so as to maintain said resilient strip in a downward-' ly concaved position with said first and second contacts in contact with each other during normal operation,

and a spring biased downward projecting member positioned above said spring resilient strip and having sufficient length when depressed into a lowered posi- 5 tion to move said resilient strip past a dead center position from an upwardly convexed to a downwardly concaved position,

strip downward past the dead center position after said resistance wire has cooled.

whereby heating of said resistance wire allows stretching of said wire, thereby alleviating said concaved deflection of said resilient strip until a dead center position is past and said resilient strip assumes its convex biased position until moved back past dead center position by said projecting member,

said resilient strip having an opening therethrough,

said projecting member having a narrow lower portion positioned to pass through the opening in said resilient strip and press against the mounting of said first contact when said projecting member is in a lowered position,

and shoulders above said narrow lower portion positioned to contact said resilient strip in a lowered position of said projecting member for moving said References Cited UNITED STATES PATENTS 3/1937 Schmidinger 200-113 10/ 1945 Berninger 2001 13 4/1953 Welter 200l22 X 10/1962 Siiberg 200113 10/ 1963 Korsgren et al 200-67 FOREIGN PATENTS 7/1954 France.

15 BERNARD A. GILHEANY, Primary Examiner.

H. A. LEWITIER, Assistant Examiner. 

